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My Packs: UWO (BIO 2382) > (Damjanovski) Lecture 7 > Flashcards

(Damjanovski) Lecture 7 Flashcards

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1
Q

Where does actin function?

A

Cortex
* Outside, close to the plasma membrane

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2
Q

Is actin more or less dynamic than tubulin?

A

More dynamic (constantly changing)

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3
Q

What structures can actin form in epithelial cells?

A
  • Microvilli
  • Cell cortex
  • Adherens belt
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4
Q

What structures can actin form in migrating cells?

A
  • Filopodia
  • Lamellipodium
  • Stress fibers
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5
Q

What functions can actin have in muscle and non muscle cells?

A
  • Phagocytosis
  • Endocytosis and transporting vesicles
  • Contratile ring (mitosis)
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6
Q

Under a microscope, how does areas with actin appear?

A

Fuzzy, indicates it is changing fast

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7
Q

Actin structure

A

Consists of 4 subunits, different in different types of actin
* alpha subunits in muscles
* beta subunits in cortex
* gamma subunits in stress fibers

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8
Q

What form of actin is the monomer?

A

G actin

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9
Q

What form of actin is the polymerized actin?

A

F actin

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10
Q

What is the structure of actin filaments?

A

Double helix
* 72 nm per spin

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11
Q

What does actin use?

A

ATP

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12
Q

What does ATP binding cleft in actin provide?

A

Polarity
* Directionality due to unequal sides

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13
Q

When decorated with myosin S1, what does actin filaments appear?

A

Pointed end (-) and barbed end (+)

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14
Q

Polymerization of actin occurs preferably at…

A

(+) end

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15
Q

Can actin polymerize at (-) end?

A

Yes, but the (+) end is faster

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16
Q

Why is actin polymerization faster at (+) end?

A

Lower critical concentration required

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17
Q

Critical concentration of actin (+) end

A

0.12 micromolarity

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18
Q

Critical concentration of actin (-) end

A

0.60 micromolarity

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19
Q

What is it called when polymerization and depolymerization are simultaneously occurring in actin?

A

Treadmilling

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20
Q

Is polymerization faster or depolymerization faster?

A

Polymerization

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21
Q

What 3 things regulate actin polymerization?

A
  1. Thymosin
  2. Profilin
  3. Cofilin
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22
Q

Thymosin

A

Sequesters actin and provides a reservoir of actin
* Binds to actin, not allowing it to be a component of critical concentration

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23
Q

Profilin

A

Promotes actin polymerization
* Changes G-ADP to G-ATP, as only ATP can polymerize

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24
Q

Cofilin

A

Enhances depolymerization

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25
Capping proteins of actin
1. CapZ (+) 2. Tropomodulin (-)
26
With cap proteins on, actin filaments cannot...
Grow or Shrink
27
Actin disrupting drugs
1. Cytochalasin 2. Phalloidin
28
Cytochalasin
Depolymerizes actin filaments * Cannot grow
29
Phalloidin
Stabilizes actin filaments * Cannot shrink
30
What acts as nucleating proteins for actin?
Formins
31
Formin activity is regulated by...
Rho GTP
32
Are formins involved in branching?
No, only unbranched filaments
33
Which end does actin polymerize when regulated by formin? What effect does this have?
At the site in contact with the nucleating factor (formin) * Polymerization will push * Depolymerization will pull
34
What does RhoGDP act as on actin filmaments?
Cap
35
What mediates actin filament branching?
Arp 2/3
36
What does Arp 2/3 activation require?
Nucelation promoting factor (NPF) * WASp or WAVE
37
What do NPFs have to be activated by?
1. WASp: Cdc43 2. WAVE: Rac
38
What can Arp2/3 dependent actin assembly aid in?
1. Branching can aid in endocytosis by pulling on membrane 2. Branching can aid in phagocytosis by pushing on membrane
39
What proteins are important for bundling actin for microvilli?
1. Fimbrin 2. Alpha-actinin
40
What protein is important is cross linking actin in the cell cortex?
Spectrin
41
What protein is important for cross linking actin in the leading edge, stress fibers, and filopodia?
Filamin
42
Besides spectrin, what other protein do red blood cells need to support the actin network?
Ankyrin
43
What actin network binding proteins do microvilli require?
Ezrin
44
What actin network binding protein do muscles need?
Dystrophin * Binds to transmembrane proteins and actin network
45
What is the motor protein for actin?
Myosin
46
What is the most abundant form of myosin?
Myosin II
47
Myosin structure
Heavy and Light chains
48
What is the head in myosin?
ATPase * Binds to actin
49
What does neck region in mysoin do?
Bind light chain
50
What does the tail region do in myosin?
Binds cargo
51
Myosin classes
Class I Class II Class V
52
Class I myosin
1 heavy chain, small tail * Binds actin to membrane * Membrane association and endocytosis roles
53
Class II myosin
2 heavy chains * Forms thick fillaments where all the heads are on 2 sides and tails are in middle * Run parallel to actin filaments * Contraction roles
54
Class V myosin
2 heavy chains, 6 light chains * Heavy chain binds directly to cargo (tail) * Moves fastest * Organelle (cargo) transport
55
Myosin moves in what direction?
(+) end
56
What can be used to detect myosin powered movement?
Sliding filament assay
57
How far does a myosin head move?
72 nm (1 rotation of the helix)
58
What is the relationship between neck region length/number of light chains to myosin movement?
Longer neck (more light chains) = Faster movement
59
# Myosin movement: The myosin head starts in...
Rigour state * Bound to ADP
60
# Myosin movement: Step 2
Release actin * Binds to ATP
61
62
# Myosin movement: Step 3
ATP hydrolysis * Head moves to (+) end in cocked state * Phosphate is still kept in head after hydrolysis
63
# Myosin movement: Step 4
Myosin head binds to actin filament
64
# Myosin movement: Step 5
Power stroke * Phosphate is released * Neck bends and head moves while attached to myosin
65
Sarcomere bands
1. A band = Myosin II filaments 2. I band = Myosin II heads to Z disk
66
What happens in the skeletal muscle sarcomere during contraction?
Z disks come closer together during contraction * I bands decrease in size * A bands stay the same
67
Why is it called the Z disk?
At the (+) end of actin filaments which is capped by CapZ
68
What are 2 proteins are important in stabilizing sarcomeres?
1. Titin 2. Nebulin
69
Titin
Holds myosin thick filaments in place * Compress when contracting * Push apart when relaxing
70
Nebulin
Coats actin * Stabilizes
71
For the actin fingers in sarcomeres to be stable, what needs to be present?
1. CapZ (+) 2. Tropomodulin (-) 3. Nebulin
72
Troponin/tropomyosin
Cover myosin binding sites on actin
73
What causes troponin/tropomyosin shifts ?
Binding of Ca2+ (from release from t tubules in sarcoplasmic reticulum)
74
What happens when troponin/tropomyosin shifts?
Myosin heads can shift and bind (contract)

My Packs: UWO (BIO 2382) (6 decks)

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